On Thu, Aug 27, 2009 at 12:54:05PM -0400, Jeff Garzik wrote:> DM has some barrier code... but the above code was pasted from DM's > make_request function, so I am guessing that DM's barrier stuff is > incomplete and disabled at present.

That code is from the new request-based multipath implementation in 2.6.31which doesn't yet.

But bio-based dm does support barriers now. (Just missing some patches tocomplete the dm-raid1 support that are still under review IIRC.)